1. Field of the Invention
The present invention relates to a method for producing biphenyltetracarboxylic esters. More particularly, the present invention relates to an improved method for producing biphenyltetracarboxylic esters from orthophthalic diesters with high selectivity and yield in a two or more-step oxidative coupling procedure in the presence of a catalyst, while the composition of the catalyst is controlled.
2. Description of the Prior Art
In Japanese Unexamined Patent Publication (Kokai) No. 55-153747, the present inventors disclosed a process for producing 3,3',4,4'-biphenyltetracarboxylic tetraesters (s-BPTT) by the selective oxidative coupling reaction of orthophthalic diesters in a molecular oxygen-containing atmosphere in the presence of a catalyst consisting of an organic palladium salt and 1,10-phenanthroline and/or .alpha.,.alpha.'-bipyridyl or a chelate compound of the above-mentioned two compounds at an elevated temperature.
Also, Japanese Unexamined Patent Publication (Kokai) No. 56-46844 discloses a similar process to that disclosed in Japanese Unexamined Patent Publication No. 55-153747, except that the molecular oxygen-containing gas is bubbled into the reaction mixture containing orthophthalic diester and the catalyst.
However, the present inventors found that in the above-mentioned processes, the yield of s-BPTT calculated based on the amount of the orthophthalic diester used is not always satisfactory and the reaction yield of s-BPTT calculated based on the amount of the palladium compound used, which is very expensive, is also not always satisfactory.
U.S. Pat. Nos. 4,292,435 and 4,338,456 disclose a similar process to that disclosed in Japanese Unexamined Patent Publication Nos. 55-153747 and 56-46844, except that the catalyst contains an organic copper salt as a catalytic component in addition to the palladium salt and 1,10-phenanthroline or .alpha.,.alpha.'-bipyridyl. However, these processes are also not always satisfactory in the yield of the resultant biphenyltetracarboxylic ester.
Accordingly, it has been strongly desired to provide a new method which is capable of selectively converting the orthophthalic diesters to the corresponding biphenyltetracarboxylic esters at a high yield based on the amount of the orthophthalic diesters used and the amount of the expensive palladium compound used.